Add isCapped() function to collection.js

1. Add a `.isCapped()` for your collection object and add test for it.
2. I found a bug while testing my code like that: `db.collection("test1").find().size()` will throw a error, hence that I must do `db.collection("test1").find().skip(0).size()`, So i updated your cursor.js, now a cursor within a default skip value, it means I can do `db.collection("test1").find().size()`